Abstract : Repeated social defeat in the rat induces long-lasting cardiovascular changes associated with anxiety. In this study, we investigated the effects of repeated social defeat on breathing. Respiratory rate was extracted from the respiratory sinus arrhythmia RSA peak frequency of the ECG in rats subjected to social defeat for four consecutive days. Respiratory rate was recorded under anesthesia six days D+10 or 26 days D+30 after social defeat. At D+10, defeated D rats spent less time in the open arms of the elevated plus maze test, had heavier adrenal glands, and displayed bradypnea, unlike non-defeated ND animals. At D+30, all signs of anxiety had disappeared. However, half of the rats still displayed bradypnea DL rats, for low respiratory rate indicated by a lower RSA frequency, while those with higher respiratory rate DH rats had recovered. Acute blockade of the dorsomedial hypothalamus DMH or nucleus tractus solitarii NTS 5-HT3 receptors reversed bradypnea in all D rats at D+10 and in DL rats at D+30. Respiratory rate was also recorded in conscious animals implanted with radiotelemetric ECG probes. DH rats recovered between D+10 and D+18, while DL rats remained bradypneic until D+30. In conclusion, social stress induces sustained chronic bradypnea mediated by DMH neurons and NTS 5-HT3 receptors. These changes are associated with an anxiety-like state that persists until D+10, followed by recovery. However, bradypnea may persist in half of the population up until D+30 despite apparent recovery of the anxiety-like state.